Table 3 Descriptive and psychometric properties of questionnaire measures (measures manual available via Oliver et al. [33])

Background Information Questionnaire

Reporting of gender, age, verbal ability, mobility, and diagnostic information of neurodevelopmental disorder (e.g. date of diagnosis, provision of diagnosis)

N/A

N/A

Wessex Behaviour Scale [32]

Proxy measure of level of ability in individuals with intellectual disability. Relevant incapacities and speech scales encompass subscales relating to incontinence, mobility, self-help, vision, hearing, speech, comprehension, and literacy.

Items are rated on a three-point scale from 1 (severe impairment) to 3 (no impairment). Self-help score is based on ability to independently: (1) wash, (2) feed, and (3) dress, with scores ranging from 3 (not able) to 9 (able).

Inter-rater reliability of the incapacities and speech scales originally reported by the authors range from 78% (self-help and literacy) to 92% (mobility) for both children and adults with intellectual disability.

Gastro-oesophageal Distress Questionnaire (GDQ [34]) ^a

Assesses for behaviours indicative of gastro-oesophageal reflux. The questionnaire consists of 12 questions relating to behaviours in the last 2 weeks (e.g. 'cough, gag, or regurgitate?') and five questions relating to lifetime behaviours (e.g. 'Does the person you care for sleep sitting or propped up?').

The first 12 questions are rated on a five-point scale from 0 (not occurred) to 4 (more than once an hour) and the five lifetime questions are a combination of yes/no responses and four-point Likert scales. For each question, a score ≥ 2 or answer of yes is indicative of a cut-off for that item (equalling 1). Therefore, the total score is derived from the number of cut-off points obtained (maximum score of 17, ≥ 5 indicative of likely reflux).

N/A

Health Questionnaire (HQ [35]) ^a

Measures the presence of 15 health conditions across the person’s lifetime (lifetime) and within the previous month (current). Of these 15 conditions, eight conditions that are highly prevalent in SAS are reported.

Associated severity scores can be calculated for both lifetime and current conditions on a four-point scale from 0 (never occurred) to 3 (severe). In this paper, only the presence of current health conditions in the SAS group (yes/no) are reported.

Good inter-rater reliability mean Kappa coefficient values are reported at item level for both lifetime (.72) and current health conditions (.76).

Social Communication Questionnaire (SCQ [36])

The lifetime version of the SCQ is used as a screening measure of autism characteristics and is validated in individuals aged 4 years and over. Formerly known as the Autism Screening Questionnaire, the 40 items are based on content from the Autism Diagnostic Interview.

Items are rated according to a yes/no response, with total scores ranging from 0 to 39 (question 1 relating to verbal ability is not included in total score calculation; a score of 0–33 is obtainable for non-verbal individuals). Three items are not grouped into subscales, the other 36 items are grouped according to communication; reciprocal social interaction; and restrictive, repetitive, and stereotyped behaviour. Cut-off scores of ≥ 15 and ≥ 22 are utilised as indicative of autism spectrum disorder and autism respectively.

Good diagnostic validity in school-age children with intellectual disability and pervasive developmental disorders, with sensitivity and specificity values of .92 and .62 respectively, when a cut-off score of ≥ 15 is utilised. Good concurrent validity reported with both the Autism Diagnostic Interview and the Autism Diagnostic Observation Schedule.

Repetitive Behaviour Questionnaire (RBQ [37])

Informant report measure of the occurrence of 19 observable operationally defined behaviours (e.g. hand stereotypy, organising objects, preference for routine) and their frequency during the previous month. Operationally defined definitions and further subscale information is provided by Moss et al. [38].

The frequency of behaviours is rated on a five-point scale ranging from 0 (never) to 4 (more than once a day). Items can be grouped into five subscales of repetitive behaviour: stereotyped behaviour, compulsive behaviour, insistence on sameness, restricted preferences, and repetitive speech. Restricted preferences and repetitive speech subscales are not calculated for individuals with limited verbal ability, therefore a maximum score of 76 can be obtained for verbal participants and 60 for non-verbal participants.

Spearman coefficients for inter-rater reliability range from .46 to .80 at item level with 73% of items above .60. Spearman coefficients test–retest reliability statistics range from .61 to .93 at item level with 52.6% of items above .80. Good concurrent validity is reported between the RBQ and the restrictive, repetitive, and stereotyped behaviour subscale of the SCQ (.60). Internal consistency is good at full-scale level (.80) and for the stereotyped behaviour and compulsive behaviour subscales (.70).

Challenging Behaviour Questionnaire (CBQ [39])

Informant report measure of the presence of self-injury, physical aggression, property destruction, and stereotyped behaviour in the last month on a yes/no basis.

A self-injury severity score can be calculated out of 14 based on the duration, response severity, and frequency of self-injury. Item scores are summed to provide an overall severity score, with higher scores denoting higher levels of self-injury severity. In this paper, stereotyped behaviour from the CBQ is not reported, as a more detailed description of stereotyped behaviour is provided by the RBQ stereotyped behaviour subscale.

Moderate to very strong Kappa coefficient values are reported for inter-rater reliability (.60 to .92), as well as good concurrent validity with the Aberrant Behavior Checklist (.56).

The Activity Questionnaire (TAQ [40])

Informant report measure comprising of 18 items relating to overactivity (e.g. ‘Does the person find it difficult holding still?’), impulsivity (e.g. ‘Does the person want things immediately?’), and impulsive speech (e.g. ‘Does the person often talk excessively?’).

Behaviour frequency is rated according to a five-point scale ranging from 0 (never/almost never) to 4 (always/almost all of the time). Items are grouped according to three subscales, with higher scores depicting greater behavioural severity: overactivity (0–36), impulsivity (0–24), and impulsive speech (0–24). Impulsive speech is not calculated for non-verbal participants.

Good mean item-level correlation coefficient values have been reported for both inter-rater reliability (.56) and test–retest reliability (.75). Inter-rater and test–retest reliability statistics are also good at both subscale and total score level (≥ .70).

Mood, Interest, and Pleasure Questionnaire—Short Form (MIPQ-S [41])

Measurement of affect, appropriate for use in individuals with intellectual disability. Six items correspond to mood (e.g. ‘In the last two weeks, do you think the facial expression of the person looked flat … ’), and six items correspond to interest and pleasure (e.g. ‘In the last two weeks, how interested did the person appear to be in his/her surroundings?’).

Behaviour frequency during the past 2 weeks are rated on a five-point scale from 0 (never) to 4 (all of the time), with subscale scores for mood and interest and pleasure ranging from 0 to 24. Higher subscale scores are indicative of more positive affect and higher levels of interest and pleasure.

This measure reports good internal consistency values for total score (.88), mood (.79), and interest and pleasure (.87), as well as good correlation coefficient values for both test–retest reliability (.97) and inter-rater reliability (.85).

Anxiety, Depression, and Mood Scale (ADAMS [42]) ^a

Informant report measure comprising of 28 items that measure internalising states relating to anxiety, depression, and mood.

Items are rated on a four-point scale ranging from 0 (not a problem) to 3 (severe problem). Items are grouped according to five subscales: manic/hyperactive behaviour, depressed mood, social avoidance, general anxiety, and compulsive behaviour. In this paper, conservative subscale cut-off scores of ≥ 9 and ≥ 10 are utilised for depressed mood, and generalised anxiety respectively, as recommended by Hermans et al. [43].

This measure is specifically validated in older adults with intellectual disability (sensitivity = .80 to .82; specificity = .65 to .78), but also demonstrates good test-retest reliability in children and adults with intellectual disability aged 10-79 years (.81).

Hospital Anxiety and Depression Scale (HADS [44]) ^a

This 14-item self-report measure of anxiety (e.g. ‘I get sudden feelings of panic’) and depression (e.g. ‘I look forward with enjoyment to things’) was originally developed for use in the general population but has since been used to measure caregiver well-being in a number of neurodevelopmental disorder studies (e.g. [45, 46]).

Items are rated on a four-point scale from 0 to 3, with higher scores denoting a greater severity of anxiety and depression symptomatology. A maximum score of 21 can be obtained on each anxiety and depression subscale, with cut-off scores ≥ 15 indicative of severe anxiety/depression and cut-off scores ≥ 8 indicative of mild symptomatology. In this paper, subscale cut-off scores of ≥ 8 are utilised, as recommended by Bjelland et al. [47].

Good specificity and sensitivity are reported for both anxiety (specificity = .78; sensitivity = .90) and depression (specificity = .79; sensitivity = .83) when a cut-off score of 8 is utilised. This measure also has good established concurrent validity (.60 to .80) when compared with standardised measures of anxiety and depression.